The present invention relates to a diagnostic system for a motor vehicle.
Recently, a motor vehicle has been equipped with an electronic control system for controlling various components of an engine, such as fuel injectors, thereby improving driveability, exhaust gas emission, fuel consumption and engine power. The electronic control system controls the components based on information represented by output signals from various sensors for detecting engine operating conditions. Accordingly, if malfunctions of components and sensors occur, the engine does not properly operate.
However, because of the complexity of the electronic control system, it is difficult to immediately find out the trouble. Accordingly, a diagnostic device for easily checking the electronic control system should be provided as equipment in an auto shop. The electronic control system has a memory and a communication system to be connected to the diagnostic device.
Japanese Patent Application Laid-Open 58-12848 discloses a diagnostic system in which a bidirectional communication system is provided between the electronic control system and the diagnostic device, thereby diagnosing data based on output signals from various sensors and control data for various actuators in the control system.
When the electronic control system is communicated with the diagnostic device and the diagnosis operation starts, the control system continuously transmits the data signal to the diagnostic device. Accordingly, the load exerted on a central processor unit (CPU) provided in the control system becomes extremely large.
If the speed of the engine increases to a high speed during the continuous data transmission from the control system to the diagnostic device, execution of control programs of the control system such as fuel injection control and ignition timing control is affected by the large load on the CPU. As a result, fuel injection and ignition timing can not be properly controlled.
In order to solve such problems, a demand signal demanding termination of the data transmission is applied from the diagnostic device to the control system. After the control system receives the data transmission terminating demand signal and interprets the termination of the data transmission, the control system stops transmitting data. Consequently, it takes time before termination of the data transmission, which may cause malfunctioning of the engine.
Further, there is another motor vehicle having a plurality of control units for controlling the engine, transmission, brake and cruise control. When these control units are diagnosed, the control units are connected in parallel to the diagnostic device through a pair of bus lines. The diagnostic device sends a particular control unit designating signal to all control units through one of the bus lines. In response to the particular control unit designating signal, each control unit determines whether the control unit itself is designated or not. A designated control unit is connected with the diagnostic device through the bus lines, and hence the other control units are excluded from the bus lines.
When the designated control unit is in communication with the diagnostic device, the control unit continuously transmits signals to the diagnostic device through the bus lines. Under such a condition, other control units are not diagnosed. In order to diagnose another control unit, the diagnostic device produces a data transmission terminating demand signal which is applied to all control units. After the control unit in communication with the diagnostic device stops transmitting the data transmission in response to the transmission terminating demand signal, the newly designated control unit is connected to the diagnostic device to start data transmission. However, since the communication procedure is complicated, it takes a long time to diagnose a plurality of control units.
Consequently, it is difficult to quickly change the communication between the control unit and the diagnostic device. Thus, diagnosis operation for a plurality of control units can not be quickly performed.